This invention relates to the art of plating and more particularly to an improved aqueous acidic plating bath, method of using the same and resulting plated alloy.
The invention is particularly applicable for barrel plating of small parts, such as screws, washers, bolts, etc., which have been first coated with a bright substrate of nickel, and it will be described with particular reference thereto; however, it is appreciated that the invention has broader applications and may be used for depositing an alloy onto various metal substrates of various small parts or workpieces.
For some time, it has been somewhat standard practice to electrolytically deposit chromium over bright metallic substrates or coatings, such as nickel, to provide a tarnish resistant, bright surface on small parts, such as screws, bolts, washers, etc. This chromium finish was not only decorative, but also protected the part from corrosion. Chromium plating is a relatively complex plating procedure which is quite expensive, especially for relatively small parts. In a chromium process, the parts must be scattered on a tray in a single layer and passed through the electrolytic plating bath at a controlled rate. If the parts have been handled or engage each other, there is a tendency to create a defect which must be visually inspected and discarded or stripped and reprocessed. Because of the demands for excellence in chromium finishing, and the lengthy procedure for applying the chromium finish, a substantial amount of time and effort is required to plate the relatively small parts with the normal plating procedure. Only relatively small quantities of these parts can be plated on the standard plating tray which requires a substantial amount of part handling and post-plating inspection. A chromium line for small parts requires a high amount of energy per part and the required equipment involves a substantial capital investment which is difficult to recover when small parts are being plated. This is especially true in the highly competitive field of plating small parts for the automotive trade. For these reasons, there has been a substantial effort to develop a substitute for the basic chromium protective and decorative coating which does not have the extensive economic burdens of chromium plating high volumes of small parts. Such a substitute process must have a somewhat close appearance to the blue-white color of chromium. One attempt to accomplish a substitute for chromium has been the use of a binary alloy of cobalt and tin. Such an alloy was found to lack the protective qualities of a chromium finish under certain circumstances. In addition, a cobalt-tin binary alloy would not generally have the same degree of abrasion resistance as chromium. Because of these deficiencies in a cobalt and tin binary alloy, there has still been a substantial demand for a true substitute for chromium. One ternary type of alloy has been suggested using indium, zinc or chromium with cobalt and tin. This type of plating system has many of the disadvantages of prior chromium plating.